1. Field of the Invention
The invention relates to articles comprising a magnetoresistive material.
2. Discussion of the Related Art
The magnetoresistance (MR) of a material is typically measured by the resistivity pH of the material in an applied magnetic field H minus the resistivity .rho..sub.o in the absence of an applied magnetic field. This resistivity difference .DELTA..rho. (equal to .rho..sub.H -.rho..sub.o) may be normalized by dividing by .rho..sub.H or .rho..sub.o, and is thereby expressed as a magnetoresistance ratio in percent. Resistance values may alternatively be used. Conventional materials such as permalloy typically have a MR ratio of a few percent when normalized by either value.
Materials exhibiting magnetoresistance ratios of greater than a few percent are useful in a variety of devices. The devices utilize the magnetoresistive materials' ability to respond, by way of resistive changes, to small changes in applied magnetic field. This effect is useful, for example, in magnetic sensing devices, current sensing devices, memory elements, or even acoustic devices. Examples of useful devices are discussed, for example, in co-assigned U.S. Pat. Nos. 5,450,372 and 5,461,308, the disclosures of which are hereby incorporated by reference.
Desirable MR has been observed in mixed metal oxides, e.g., La--Ca--Mn--O, La--Ba--Mn--O, and La--Sr--Mn--O. See, e.g., K. Chahara et al., Applied Physics Letters, Vol. 63 (14), at 1990; R. von Helmholt et al., Physical Review Letters, Vol. 71 (14), at 2331; and co-assigned U.S. Pat. Nos. 5,549,977 and 5,538,800. The magnetoresistance of La-Sr-Mn-O perovskites appears to be better in polycrystalline samples, as opposed to single crystals, possibly due to spin-polarized tunneling of electrons between grains. See H. Y. Hwang et al., Physical Review Letters, Vol. 77 (10), at 2041. In particular, it has been found that trilayer structures using La--Ca--Mn--O and La--Sr--Mn--O perovskites undergo a change in resistance by a factor of 2 in a low applied field of 200 Oe, indicating the potential use of such materials in field sensors. See J. Z. Sun et al., Applied Physics Letters, Vol. 69 (21), at 3266.
These manganite perovskites, however, have a relatively low Curie temperature, the highest achieved being about 105.degree. C. See, e.g., H. Y. Hwang et al., Physical Review Letters, Vol. 75 (5), at 914. The Curie temperature is the temperature above which ferromagnetism disappears. As the temperature moves above and beyond the Curie temperature, magnetoresistance becomes smaller. Devices using such manganite perovskites must therefore be placed in a working environment below the Curie temperature to ensure that the requisite magnetoresistive properties are present. Clearly, magnetoresistive materials having a Curie temperature higher than 105.degree. C. offer commercial potential for a wider range of applications. In addition, the manganite perovskites are relatively difficult to prepare and incorporate into devices such as those discussed above, compared to more conventional metals and oxides.
Thus, magnetoresistive materials that have higher Curie temperatures than current magnetoresistive materials, and which can be produced and incorporated into devices more easily, are desired.